NOM is seeking damages as a result of the unauthorized disclosure, according to a complaint filed by the ActRight Legal Foundation on behalf of NOM in the US District Court for the Eastern District of Virginia.

NOM is also asking that the IRS disclose the person or persons responsible for the release of the organization's confidential tax return information. The unauthorized disclosure of tax information is a felony punishable by up to five years in prison.

The lawsuit claims that on March 30, 2012, NOM became aware that the unredacted Schedule B to its Form 990, containing the names and addresses of its donors, was illegally inspected and disclosed by an employee or employees of the IRS, who then passed the information on to HRC. According to the lawsuit, HRC shared the tax return information with the Huffington Post.

"This is a chilling set of circumstances that should ring alarm bells across the nation," NOM President Brian Brown said in a written statement.

In a March 30, 2012, blog post on the Huffington Post website, HRC Assistant General Counsel Darrin Hurwitz wrote that in the aftermath of California's Proposition 8, NOM strategically leveraged large secret donations to fund the anti-gay marriage movement across the country. In his blog, Hurwitz published the names of some organizations and individuals and the amounts they donated to NOM for its Prop 8 effort.

NOM also claims in its lawsuit that HRC published the confidential tax return information on its website on or about March 30, 2012. According to the lawsuit, HRC has since removed that information from its site.

In another article posted on the Huffington Post site on March 30, 2012, one of NOM's major donors was the political action committee (PAC) for then-Republican presidential candidate Mitt Romney. The PAC made a $10,000 donation to the not-for-profit conservative group in 2008.

Included in the article is a link to a PDF of NOM's Schedule B, which includes the names and addresses of NOM donors, the Huffington Post said it received from HRC. HRC stated it received the document from a "whistleblower." The Huffington Post has refused to take the article off of its website.

In a press release dated March 30, 2012, HRC also confirmed Romney's PAC donation to NOM, based on "never-before-seen financial documents from the National Organization for Marriage."

HRC claimed it reviewed copies of Romney's Free and Strong America PAC filings with the Federal Election Commission, and no contribution to NOM was disclosed in 2008.

"However, knowing that oftentimes federal leadership PACs create state PAC entities in places where contribution limits do not exist or disclosure laws are lax, HRC did discover an Alabama-based Free and Strong America PAC that in 2008 does disclose the $10,000 contribution to NOM," HRC stated in the release.

HRC also cited an October 28, 2008, article in Utah's Deseret News in which a Romney spokesperson claims a contribution to Prop 8 was made, but there is no evidence of disclosure of a contribution to the effort.

"If Romney's spokesperson is accurate, then both Romney's PAC and NOM may have violated California disclosure laws," HRC stated.

NOM Chairman John Eastman testified on June 4, 2013, before the House Ways and Means Committee about the leak of the organization's confidential tax return information. He said NOM filed written requests for investigation on April 11, 2012, with both the Treasury Inspector General for Tax Administration (TIGTA) and the US Department of Justice.

"NOM noted in those requests its belief, based on the computer forensic evidence it had uncovered, that the IRS had to be the source of the disclosure," Eastman said. "TIGTA responded in an April 20, 2012, letter that acknowledged receipt of NOM's request and provided NOM with a complaint number, Number 63-1204-0051-C. At the outset of the investigation, investigators from TIGTA sought to determine whether the document with the internal IRS markings might have originated from NOM, but once NOM demonstrated that it had not, we received no further information about the investigation. As of May 31, 2013, however, over a year after making the request, NOM has not received any information resulting from the requested investigation from TIGTA."

Eastman also stated that NOM filed a series of Freedom of Information Act requests in August 2012, seeking to find out the status and results of the TIGTA investigation. He claimed the IRS and the TIGTA declined to provide NOM with the most relevant information. In a response dated May 3, 2013, Eastman said the TIGTA declined to acknowledge the existence of the investigation, even though NOM was provided with a complaint number.

"Worse, that latest nonresponsive 'response' from TIGTA even adopts the Orwellian position that the same statute which prohibits the disclosure of a taxpayer's confidential tax return information prevents any disclosure of the culprit of that felony," Eastman told Congress.

Eastman's testimony came less than a month after the IRS apologized for scrutinizing the tax-exempt status of groups with conservative titles, such as "Tea Party" or "Patriot," in their names.

"So far, the IRS and Obama administration officials have stonewalled our every attempt to get information or answer basic questions about who knew what inside the IRS and who in the Obama administration had any information or involvement in this crime," Eastman said in a written statement. "Our lawsuit will be a powerful tool in ending the stonewalling and getting the truth to the American people."